1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, relates to a secondary battery using an organic radical compound for an electrode active material and repeating charge and discharge by making use of a battery electrode reaction of the electrode active material.
2. Description of the Related Art
With the market expansion of portable electronic devices such as cellular phones, laptop computers, and digital cameras, secondary batteries with high energy densities and long lifetimes have been demanded as codeless power sources for these electronic devices.
Then, in order to meet the demands, secondary batteries have been developed which use alkali metal ions such as lithium ions as charged carriers and make use of an electrochemical reaction involving transfer of the charges. In particular, lithium ion secondary batteries with high energy densities have been now used widely.
Among elements constituting a secondary battery, electrode active materials (a positive electrode active material and a negative electrode active material) refer to materials which contribute directly to battery electrode reactions of a charge reaction and a discharge reaction, and play a central role in the secondary battery. In addition, in a lithium ion secondary battery, a lithium-containing transition metal oxide and a carbon material are used respectively as the positive electrode active material and the negative electrode active material, and charge and discharge are carried out by making use of a reaction for intercalating lithium ions into and a reaction for deintercalating lithium ions from these electrode active materials.
However, the lithium ion secondary battery has the problem of the limited charge and discharge rate, because the lithium ion transfer at the positive electrode is a rate-limiting step. More specifically, in the lithium ion secondary battery described above, the transfer rate of the lithium ion is slow in the transition metal oxide of the positive electrode as compared with its electrolyte and negative electrode, and the rate-limiting battery reaction at the positive electrode limits the charge and discharge rate, resulting in limits to the increase in output power and to the reduction in charging time.
Therefore, in order to solve this problem, the research and development of secondary batteries using an organic radical compound as a reactant or a product of an electrode reaction have been actively carried out in recent years.
The organic radical compound includes a radical that is an unpaired electron in the outermost shell of the electron orbital. Although this radical is generally a reactive chemical species, and many of radicals will disappear after a certain period of lifetime due to the interaction with the surrounding material, the radical will be stabilized depending on the resonance effect, the steric hindrance, and the solvation state.
In addition, the radicals have a fast reaction rate, and thus allow the charging time to be completed within a short period of time by carrying out charge and discharge with the use of the redox reaction of the stable radicals. In addition, the organic radical compound has reactive unpaired electrons localized in radical atoms, and thus allows the concentration of the reaction site to be increased, thereby allowing high-capacity secondary batteries to be expected to be achieved.
Furthermore, it is believed that the organic radical compound allows an excellent secondary battery to be obtained which has a cycle characteristic independent of diffusion of the electrode active material and is thus superior in terms of stability, because only the radicals contribute to the reaction. In addition, the organic radical compound generally has, as components, elements with a small atomic weight such as carbon, hydrogen, oxygen, and nitrogen, and thus also allows a high-capacity secondary battery to be obtained while achieving reduction in the weight of the battery.
Further, for example, Patent Document 1 proposes an active material for a secondary battery, which participates in an electrode reaction of the secondary battery, and the reactant or product of the active material in the electrode reaction is a neutral radical compound.
Patent Document 1 uses, as the electrode active material, a nitroxyl radical compound, a nitroxyl nitroxide radical compound, an oxyradical compound, a nitrogen radical compound, etc., and carries out charge and discharge by making use of the redox reaction of the radicals. In addition, as described above, the radicals have a fast reaction rate, and thus lead to high outputs and also allow charging to be completed within a short period of time.
In addition, the secondary battery in Patent Document 1 has a stack structure with a positive electrode current collector 101, a positive electrode layer 102, a separator 103 including an electrolyte, a negative electrode layer 104, and a negative electrode current collector 105 sequentially stacked, as shown in FIG. 4, and the electrode active material of at least one of the positive electrode layer 102 and the negative electrode layer 104 contains a radical compound.
Furthermore, Patent Document 2 proposes a secondary battery using an electrode composed of a polyradical compound layer formed on an active material layer as a positive electrode and using metal lithium as a negative electrode.
In a lithium ion secondary battery, typically, in order to electrically insulate a positive electrode and a negative electrode, a gel electrolyte and a porous separator composed of polyethylene or the like are interposed between the both electrodes, as described in Patent Document 1. However, in the case of using metal lithium for the negative electrode, the thus configured battery may have, for example, dendrite (a reduction product resulting from lithium ions) at the surface of the negative electrode, caused by repeated charge and discharge, and the growth of this dendrite may lead to a breakdown of the separator, thereby causing short circuit between the positive electrode and the negative electrode.
Therefore, in Patent Document 2, as shown in FIG. 5, the surface of an active material layer 112 formed on a current collector 111 is coated with a polyradical compound such as a nitroxyl radical compound, which serves as an insulator, in such a way that the current collector 111, the active material layer 112, and the polyradical compound layer 113 form a positive electrode 114, and metal lithium to serve as a negative electrode 115 is stacked on the positive electrode 114.
More specifically, in Patent Document 2, the polyradical compound which serves as an insulator is provided on the top layer of the positive electrode 114, and the polyradical compound layer 113 is brought into contact with the negative electrode 115 to eliminate the separator, thereby preventing any short circuit from being caused by the generation of dendrite at the negative electrode 115, even in the case of using lithium metal for the negative electrode 115.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2004-207249 (claims 1, 3, 17, 20, and 30; and FIG. 2)    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2007-157496 (claims 1 and 2; and FIG. 1)
However, Patent Document 1 has the problem of high manufacturing cost, because the separator 103 including the electrolyte is interposed between the positive electrode layer 102 and the negative electrode layer 104 while the neutral radical compound is used for the electrode active material. In addition, while this type of secondary battery is assembled by encapsulating an electrolytic solution, the electrolytic solution is also costly, and thus leads to further soaring cost of manufacturing. Moreover, the electrolytic is flammable, and thus also problematic in terms of safety.
In addition, while the polyradical compound layer 113 constituting the positive electrode 114 is brought into contact with the negative electrode 115 to eliminate the need for the separator in Patent Document 2, an electrolytic solution has to be encapsulated for use, as in the case of Patent Document 1, thus leading to high cost, and the electrolytic solution requires careful handling, and is thus also problematic in terms of safety.
In addition, in the case of using no electrolytic solution as described in Patent Document 2, a solid electrolyte such as a polymer electrolyte is interposed between the positive electrode and the negative electrode. In this case, the contact at the interfaces between the solid electrolyte and the electrodes is critically important, thus leading to a problem with the contact at the electrode-solid electrolyte interfaces.